Fix the algorithm for decoding the body length

See: https://docs.oasis-open.org/mqtt/mqtt/v3.1.1/errata01/os/mqtt-v3.1.1-errata01-os.html#_Toc442180732

Use asynchronous methods for blocking operations

Otherwise, Worker Threads from the .NET Threadpool are blocked while waiting for the next client message, and while waiting for the next message to send.

fixed: Compile Source Code Occur Errors On Mac Machine

Serialized certificates loaded as X509Certificate2

Instances of X509Certificate2 passed into an MqttClientOptionsBuilder lose data and functionality by being loaded as an X509Certificate.